| The 660 km discontinuity, the lower boundary of the earth's upper mantle, is studied through its effect on P-waves that partly convert to S-waves upon crossing the discontinuity. The converted waves are difficult to detect in individual seismograms, although a few have been observed in seismograms recorded in the Netherlands by the NARS array. In contrast, stacks of seismograms presented in this thesis, show no unambiguous converted phases. The paradox can be explained by relief on the discontinuity. We have used a Kirchhoff-Helmholtz approach to model the effects of relief and we have reached the conclusion that 30 km variations in the depth of the discontinuity can exist over distances of a few hundreds of km.; The upper mantle beneath North America is tomographically imaged using the waveforms recorded in digital seismograms from various broad-band networks. The waveforms of the fundamental mode surface wave, the higher modes and the S-wave, all of which travel through the upper mantle, are synthesized. The fit of a synthetic to the recorded seismogram quantifies the average structure of the upper mantle between the earthquake and the recording station. We have also investigated the information contained in the amplitudes of the waveforms as well as the effects of the earthquake source. The combination of path-averaged structures yields a three dimensional image of the upper mantle. The image shows that east of the North American Cordillera S-waves travel relatively fast down to depths around 250 km, which delineates this region as a cold, solid part of the upper mantle. Likewise, west of the Cordillera the continent appears to be atop a hot, viscous upper mantle. Among many other interesting features, we find an elongated high-velocity anomaly at deeper levels in the upper mantle, which can be identified as the Cenozoic remains of the subducted Farallon plate. |
